1. An immunohistological study focuses on the characteristics of T-cells, B-cells and macrophages in a phT in order to clarify the aging dependent manners of a phT.Anti-DRC positive cells and CD21 positive cells presumably belonged to follicular dendritic cells were amounted in germinal centers of a phT.The cell clusters in a phT were more increased in a child than an adult. The anti-Ki67 antibody positive staining cells, characteristic of proliferating cells were more remarkably staind in the germinal centers of a phT in child and adult.2. A one-step PCR system was developed for the detection of Epstein-Barr Virus (EBV), using a single pair of primers that were complementary to both sequences coding the EBV nuclear antigen (EBNA)-2, and encompassing a large deletion in the sequence of type 1 EBV.EBV genome was present in low consentration in phTs of children, but it was detected in high consentration in mouthwashings from adults.3. Apoptosis is a morphological involution caused by a celluar suicide program, which is associated with programd cell death. The Fas antigen mediated apoptosis and the death of immunocytes in lymphoid tissues were induced through the apoptotic mechanisms. The study was desinged to investigate the apoptotic cells in human phT by using RT-PCR technique with the primers, which were complementary to the Fas antigen. The number of apoptotic cells and Fas-positive cells changes with age, suggesting a close relationship between apoptosis and morphological and immunologic involutions of the phT.